A metal case consists of a inner case and an outer case, the inner case and the outer case have respective rear surface portions which are mutually separated but overlapped on a back side of the metal case, an inflator has fitting bolts which penetrate the rear surface portions of the inner case, the inflator secured to the rear surface portion of the inner case by fitting bolts and nuts, and top ends of the fitting bolts are covered with the rear surface portion of the outer case. Accordingly, even when a passenger pushes or touches a seat back above the fitting bolts, he or she feels smooth or pleasant to the touch.